The present invention relates to a process for producing a free hydroxylamine aqueous solution, in more detail, a process for producing a free hydroxylamine aqueous solution, in which hydroxylammonium sulfate and an alkali are caused to react with each other for obtaining free hydroxylamine, comprising the steps of adding a lower alcohol for precipitating the sulfates produced in the reaction, to separate and remove them, separating the lower alcohol by distillation, and obtaining a free hydroxylamine aqueous solution by distillation from the remaining solution.
Hydroxylamine is industrially widely used as a medical and agricultural intermediate raw material or as a metal surface treating agent, etc.
However, since free hydroxylamine is very unstable and is easily decomposed, it is generally converted into a relatively stable hydroxylamine salt (hydroxylammonium salt) for use. The decomposition of free hydroxylamine is especially likely to occur, for example, in the presence of heavy metal ions, at a high concentration, in a strong alkali or at a relatively high temperature.
When hydroxylamine is actually used in the above applications, free hydroxylamine, not a hydroxylammonium salt, is preferably used, and an aqueous solution containing free hydroxylamine at a higher concentration is often required. Such a free hydroxylamine aqueous solution has been obtained by several methods.
For example, in U.S. Pat. Nos. 4,147,623, 4,202,765, 5,788,946 and 5,872,295, a free hydroxylamine aqueous solution is liberated from a hydroxylammonium salt aqueous solution containing impurities by using an ion exchange resin. However, the hydroxylamine concentration in the obtained aqueous solution is low, and hydroxylamine expedites the degradation of the ion exchanger, to cause much temporal and economical waste.
Furthermore, as described in German Patent Publication No. 3347259, there is also a method of obtaining a free hydroxylamine aqueous solution by electrodialysis in an electrolyte from a hydroxylammonium salt aqueous solution contained in an electrolytic cell having a semipermeable membrane, but the method is technically complicated and low in yield. Furthermore, the equipment cost is high. So, the method is not yet established industrially.
A recent method for obtaining free hydroxylamine uses the reaction between a hydroxylammonium salt and an alkali. This reaction system contains a large amount of a salt formed by the anions derived from the hydroxylammonium salt and the cations derived from the alkali. So, various attempts are made to separate the salt and free hydroxylamine, for obtaining more highly pure free hydroxylamine at a higher concentration.
In the methods described in U.S. Pat. Nos. 4,707,294, 4,551,323, 4,551,324 and 4,956,168, an alcohol solution of free hydroxylamine is obtained by removing the sulfates precipitated in the reaction between hydroxylammonium sulfate and an alkali or alkali alcoholate in an alcohol. This alcohol solution is then mixed with a reagent such as an acid other than sulfuric acid or a ketone, to be used for synthesizing a hydroxylammonium salt other than hydroxylammonium sulfate or an oxime, etc. German Patent Publication No. 1247282 discloses a method in which an alcohol solution of free hydroxylamine is obtained by removing the ammonium sulfate precipitated in the reaction between hydroxylammonium sulfate and ammonia in an alcohol. However, these reactions in an alcohol are low in productivity and yield since the solubility of hydroxylammonium sulfate is low, and to improve the disadvantage, complicated equipment such as a powerful stirrer is required for the reaction system. Furthermore, in these methods, since the salt dissolved in the alcohol solution remains in the product, the purity of the product comes into question.
In German Patent Publication No. 3601803 (Japanese Patent Laid-Open (Kokai) No. Sho62-171905 and U.S. Pat. No. 4,778,669), as described above, an alcohol solution of free hydroxylamine is obtained by removing the ammonium sulfate precipitated in the reaction between hydroxylammonium sulfate and ammonia in a lower alcohol. Furthermore, a hydroxylamine stabilizer and water are added to the alcohol solution, and the alcohol is separated by distillation, to obtain a free hydroxylamine aqueous solution. However, since this method is also a reaction in an alcohol, the productivity and yield are low because of the low solubility of hydroxylammonium sulfate, and to improve the disadvantage, complicated equipment such as a powerful stirrer is required for the reaction system. Moreover, since the aqueous solution as a product is the residue of distillation, coloration and the ingress of ammonium sulfate and heavy metals come into question.
It can be seen that a consistent reaction using an alcohol solvent only is not preferable in view of productivity, yield, equipment and quality.
In the method disclosed in German Patent Publication No. 3528463, hydroxylammonium sulfate with a low ammonium ion content and the oxide and/or hydroxide of calcium, strontium or barium are caused to react with each other in water, and the alkaline earth metal sulfate is separated, to obtain a free hydroxylamine aqueous solution. Since the alkaline earth metal sulfate is generally low in the solubility in water, the grain size is extremely small, and it is difficult to industrially separate the solid. Furthermore, in this method, no other operation than precipitation is attempted for separating sulfates, and the obtained aqueous solution contains sulfates by amounts corresponding to the solubilities. By the way, as for the hydroxylammonium sulfate used for this reaction, the hydroxylammonium sulfate aqueous solution as the raw material must be crystallized several times, to decrease the ammonium ion content beforehand. Furthermore, the strontium compounds and barium compounds are relatively expensive and highly toxic. So, this method is not appropriate for industrial production.
In U.S. Pat. Nos. 5,472,679 and 5,266,290, hydroxylammonium sulfate and the oxide and/or hydroxide of an alkali metal or an alkaline earth metal are caused to react with each other in water not containing any alcohol, to obtain a reaction solution containing free hydroxylamine, and from the reaction solution, a highly concentrated free hydroxylamine aqueous solution not containing any alcohol is obtained by simple distillation at 65xc2x0 C. or lower. The sulfates precipitated in this reaction are removed before distillation, but since the aqueous solution contains much salts dissolved, much salts are precipitated in the residue in the subsequent simple distillation operation, making the simple distillation operation difficult. Furthermore, since free hydroxylamine with a boiling point higher than that of water is deposited on the precipitated salts, the yield of free hydroxylamine is extremely lowered. So, the method is not industrially advantageous.
In German Patent Publication No. 19547758 (U.S. Pat. No. 5,837,107), hydroxylammonium sulfate and an alkali are caused to react with each other in water not containing any alcohol, and the precipitated salt is removed. Then, the reaction solution is supplied to an intermediate stage of a column, and separated into a hydroxylamine aqueous solution not containing the salt discharged from an upper stage and an aqueous solution containing the salt discharged from a lower stage. In this case, the amount of the water or steam supplied from a lower stage countercurrently must be large for effecting distillation at a high temperature of 80xc2x0 C. or higher for easier distillation operation and for decreasing the discharge of free hydroxylamine from the lower stage to raise the yield, but it lowers the hydroxylamine concentration of the free hydroxylamine aqueous solution obtained from the column top. So, to achieve a necessary free hydroxylamine concentration, in most cases, subsequent concentration is necessary. Thus, the method is not industrially advantageous.
Japanese Patent Laid-Open (Kokai) No. Sho55-71609 proposes a method for obtaining highly pure crystalline hydroxylammonium sulfate. In this method, a raw solution obtained by introducing sulfurous acid gas into an aqueous solution of a sulfite and an acid sulfite is heated and hydrolyzed, to produce a mixture consisting of hydroxylammonium sulfate, ammonium sulfate and sulfuric acid according to the Raschig""s method, and the mixture is adjusted in pH. To the solution, an alcohol is added, and a solid mainly composed of ammonium sulfate is separated. From the residue, the ammonium sulfate, sulfuric acid and other impurities contained in the raw material are removed, to obtain a transparent solution. To the solution, sulfuric acid is added again to obtain highly pure crystalline hydroxylammonium sulfate. While the present invention relates to a process for obtaining free hydroxylamine from hydroxylammonium sulfate, particularly a process for obtaining highly pure free hydroxylamine at a high concentration at a high yield, the invention disclosed in Japanese Patent Laid-Open (Kokai) No. Sho55-71609 describes an impurities removing means for obtaining highly pure crystalline hydroxylammonium sulfate from a special raw mixture containing a large quantity of impurities. So, the latter invention is quite different from the present invention in object and method.
The object of the present invention is to solve the above problems, by providing a process for obtaining a highly pure and highly concentrated free hydroxylamine aqueous solution substantially not containing any salt or organic substance at a high efficiency.
In general, if a hydroxylammonium salt is treated by an alkali, a salt is produced in a large amount. In this invention, this salt is decreased beforehand, to simplify the subsequent distillation operation. Furthermore, since the lower alcohol mixed for the purpose of precipitating the salt can be obtained together with water by distillation, there is no danger of inflammability, and the recovered distillate can be recycled for effective use preferably in view of environmental conservation.
Other objects of this invention will be clarified in the following description.
This invention intended to achieve the above objects is a process for producing a free hydroxylamine aqueous solution, comprising the steps of letting an aqueous solution or slurry containing hydroxylammonium sulfate and an alkali react with each other, to obtain an aqueous solution or slurry (reaction solution) containing free hydroxylamine and dissolved or partially precipitate sulfates; mixing the obtained reaction solution and a lower alcohol, to precipitate solid sulfates; separating a solution containing the lower alcohol and some of water by distillation from the aqueous solution remaining after separating and removing the solid sulfates; and obtaining a free hydroxylamine aqueous solution from the remaining solution by distillation under reduced pressure.
The process for producing a free hydroxylamine aqueous solution of this invention includes the following preferable modes.
The sulfates produced by the reaction between hydroxylammonium sulfate and an alkali can be mostly precipitated by mixing a required amount of a lower alcohol with the reaction system, and can be separated and removed as solid sulfates.
The mixed lower alcohol can be separated by distillation from the reaction solution, and recovered and recycled for re-use in the reaction system advantageously in view of economy and environmental conservation.
After the solid sulfates are separated, the hydroxylamine deposited on the sulfates is washed away by a lower alcohol. The lower alcohol containing hydroxylamine obtained by this operation can be re-added to the reaction system for recycling.
To prevent the decomposition of free hydroxylamine, a stabilizer is added into the system containing free hydroxylamine.
The free hydroxylamine aqueous solution can be obtained safely as a highly pure product substantially not containing any salt because of distillation under reduced pressure. Furthermore in this invention, the residue remaining after obtaining the product by distillation can be recycled in the reaction system to greatly improve the yield of the reaction system as a whole, and even if the residue is recycled, the purity of the product is not substantially lowered.